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  1. #1
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    Too old to ride plastic

  2. #2
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    Quote Originally Posted by velodog View Post
    Apparently he doesn't understand MOI. The only time it is relevant is when speed changes: lower MOI wheels accelerate faster with the same power input, and slow more quickly when you coast. Otherwise, weight is weight and so MOI has NO effect on climbing. Two wheels of the same weight with different MOI (e.g. lighter hub, heavier rim) will climb the same. His reference to "pedal acceleration" is basically nonsense: lower MOI wheels will accelerate and decelerate more quickly, so it cancels out. Lower MOI wheels may feel better to a rider or they may feel more squirrely.

    This guy is repeating platitudes and should take a physics class so he understands the issue.

  3. #3
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    Ah, the usage of scientific "mumbo jumbo" in the process of salesmanship......
    "L'enfer, c'est les autres"

  4. #4
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    I had a moment, ... but it passed.
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  5. #5
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    Quote Originally Posted by Kerry Irons View Post
    Apparently he doesn't understand MOI. The only time it is relevant is when speed changes: lower MOI wheels accelerate faster with the same power input, and slow more quickly when you coast. Otherwise, weight is weight and so MOI has NO effect on climbing. Two wheels of the same weight with different MOI (e.g. lighter hub, heavier rim) will climb the same. His reference to "pedal acceleration" is basically nonsense: lower MOI wheels will accelerate and decelerate more quickly, so it cancels out. Lower MOI wheels may feel better to a rider or they may feel more squirrely.
    Exactly. I am reminded here of the auto signature of our very own DCGriz, an experienced wheelbuilder on our forum:

    Quote Originally Posted by dcgriz View Post
    With bicycles in particular, you need to separate between what's merely true and what's important.
    “Statistics are like bikinis. What they reveal is suggestive, but what they conceal is vital.” -- Aaron Levenstein

    "Beware of geeks bearing formulas."
    -- Warren Buffett

    "Education is what you get when you read the fine print. Experience is what you get when you don't. -- Pete Seeger



  6. #6
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    Quote Originally Posted by velodog View Post
    This web article was actually fascinating to me. It may exaggerate the benefits of reducing rotating weight, and if that's all one sees, OK. Throw tomatoes.

    On the other hand, it explains in detail the concept of rotating vs fixed weight, how to measure moment of inertia yourself, including all the math and even a includes a downloadable spreadsheet.

    The article also gives a decent example of the bottom line - does it matter? This is shown in the sample calculation showing how 2 actual wheels compare in accelerating from 0 to 18 MPH - one wheel uses 3.1 W, the other 1.7 W. So for 2 wheels, the saving is about 3W.

    Most people may dismiss this 3W as peanuts. And perhaps it is, but the article does give one the tools to actually measure and compare 2 wheels with a parameter other than weight alone.

    Which I take as the main point of the article. As a shopper, if you ever consider weight when buying wheels, you should also consider the moment of inertia. Because the energy to accelerate a typical rotating bike wheel is about the same and the energy needed to accelerate it forward. If weight is important, so is moment of intertia.

    And sure, you may get back those 3W you lost above, if you decelerate by coasting to a stop. But if you use your brakes to stop, which is actually common when dealing with sharp turns, intersections and red lights, you won't. Those acceleration watts are lost in braking.

    Another point about wheel moment of inertia not mentioned in this article is the "gyroscope" effect. If you hold a spinning wheel and try to move the axle up and down, you'll feel the forces generated by this effect. The larger the moment of inertia, for the same rotational speed, the stronger the effect.

    How much does the gyroscope matter, and how much can you feel it?
    I can't say, but perhaps that's part of what people are experiencing with light versus heaving wheels. Especially when moving the wheel laterally, like rocking a bike while climbing.

  7. #7
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    Quote Originally Posted by Z'mer View Post
    How much does the gyroscope matter, and how much can you feel it?
    In a round about way the article did address gyroscopic effects when it talked about the handling of wheels with a lower MOI.

    It doesn't change the fact that the author of the article doesn't understand the physics. If you think that 3W is somehow significant over the length of a daily ride, then clearly you fit the target audience for this article.

  8. #8
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    Quote Originally Posted by Kerry Irons View Post
    It doesn't change the fact that the author of the article doesn't understand the physics. If you think that 3W is somehow significant over the length of a daily ride, then clearly you fit the target audience for this article.
    You clearly have strong views on rotating weight. I have curiosity in this area, so enjoyed the article as an opportunity to understand bike wheel MOI in more detail and better educate myself.

    I think you are being unfair to the author. It's clear most of the article is derived from research and publications done by others. And those links are provided, and they all work. He may have misinterpreted or misstated some points made by others, which you then highlight to dismiss the article and author.
    Like the Campy 1987 white paper on fluid dynamic wheels.
    https://u19734227.dl.dropboxusercont...idDynamics.pdf
    This article clearly states "continuously accelerating" with heaver wheels requires "greater (pedal) effort".
    I also enjoyed reading all the other links, which give a pretty good overall picture of the subject, including the 2008 MOI test of 80 wheels
    Great wheel test 2008 – Part 2 – Inertia | Roues Artisanales

    the velo news article, which has some misguided numbers to say the least from one response
    Technical Q&A with Lennard Zinn: The great rotating-weight debate | VeloNews.com

    And the final link with validation of a model for cycling power, which gives the results for a model versus test data for losses in rolling resistance, chain friction, wheel bearings, aero drag, etc.
    https://u19734227.dl.dropboxusercont...%20cycling.pdf

    My point is this - MOI may be interesting to others trying to better understand why one set of wheels feels different than another. And it's significance may not be only small watts saved in acceleration. On the other hand, it may have no significance to you at all. Read up, and understand better.

  9. #9
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    Quote Originally Posted by Z'mer View Post
    Read up, and understand better.
    It may come as a surprise to you, but there are people here who have been thinking about and studying this issue for more than 15 years. They're worth listening to.

  10. #10
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    Back when I used to rally cars, a "polar moment of inertia" was a slang term for spinning your car off a snowy road....
    "L'enfer, c'est les autres"

  11. #11
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    Quote Originally Posted by asgelle View Post
    It may come as a surprise to you, but there are people here who have been thinking about and studying this issue for more than 15 years. They're worth listening to.
    Yes, 15 years is a long time, for some folks.

    OTOH, back when, I was riding with the college Velo Club training on tubular wheels and sewup tires. At night, I was calculating, manually, MOI for my junior year engineering dynamics class. On paper with a limited calculator.
    I was riding tubulars, racing bikes, and understanding the physics of MOI, earning a mechanical engineering degree 24 years before "some people" were "thinking about this issue".

    Those high priced calculators were introduced to the market a year or so earlier. Before that we used slide rules. As I earned my ME degree, I worked in Detroit at Ford MoCo for 2 summers, and when back at college was a leader in the design competition for "first" Mini Baha competition in FLA at FIT. Yes, there were pictures.

    Um, that over 40 years ago, in the time frame of 1974-1978.

    And so, I feel Ric, the author of the referenced article, also deserves some respect here, and so I stick up for him.
    Why?
    I get a strong feel he does this not to make a buck, but to advance the craft of wheel building, which he obviously has contributed to for a long, long time. Well before I was riding. He deserves it, and I hate to see folks bash him over tiddly shirt stuff.

  12. #12
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    Quote Originally Posted by Z'mer View Post
    OTOH, back when, I was riding with the college Velo Club training on tubular wheels and sewup tires. At night, I was calculating, manually, MOI for my junior year engineering dynamics class. ...
    The question is what are people doing with MOI. It's easy enough to calculate, but what then? Just power to overcome gravity scales with speed and aero drag scales with speed cubed, rotational inertia goes as acceleration, a much smaller quantity.

    I chose 15 years not because that was the dawn of engineering, but because it roughly corresponds to when Kraig Willett posted his article on wheel performance, Wheel Performance. That's where he showed a 50% decrease in wheel weight required ~0.4% decrease in power for a climbing course while a 50% drop in wheel MOI lowered the power by only ~0.004% (2 orders of magnitude less). For a spiky Crit the numbers are 0.2% and 0.02%; still an order of magnitude difference. Thus any wheel design which improved weight or drag would easily offset any cost due to an increase in MOI.
    Last edited by asgelle; 06-23-2017 at 04:32 AM.

  13. #13
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    Quote Originally Posted by Z'mer View Post
    You clearly have strong views on rotating weight.
    Not really. But rather than reading marketing materials, I have worked out the physics. It took a bit of refreshing but in the end it is really straightforward.

    Please note that the Campy white paper either intentionally misrepresents what is going on or ignores that the constant micro acceleration/decelerations of the pedaling effort exactly cancel out from a physics standpoint.

    Feels different is all well and good and I specifically addressed this in my comments about handling. But I'll repeat myself: MOI only counts during acceleration/deceleration. Wheels of the same weight, with different MOIs, will climb at the same speed.

    It's not strong views. It's physics.

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